New color problem in CRT TV

Discussion in 'General Electronics Chat' started by tpny, Jun 26, 2014.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    I have a new problem after I discovered the green raster transistor on the neckboard was loose at the solder joints. After that was fixed and picture has normal colors, I poked around the neck board with my mulitmeter, I took voltage measurements of the 3 raster transistor's emitter, collector, base, then I measure the 4 wires going into the funnel end of the deflection yoke, one of the wires connector when touched by probe made a staticky sound and multimeter reads overload.. I don't know if that was the cause but moments later when I turned around to check the picture it became black and white (or should I say some 2 toned colors). I still have color because turning the pots on the neckboard changes the 2 tones into 2 tones of green, blue, red respectively. And I have color on the on screen display text characters.

    My measurements at emitter, collector, base are 13Vdc, 150Vdc, 13Vdc for all three colors. They are hot to touch. I think that means they are in saturation all the time.. maybe they are fried.. Do you know what voltage should the base have normally? I think 13V reading is too huge isn't it? Thus rgb colors in picture are always in max.. The raster transistors are Toshiba C2068
    http://www.alldatasheet.com/datasheet-pdf/pdf/108529/TOSHIBA/2SC2068.html

    Or the chroma chip is toast? The chroma chip is TA... in photo attached. Please advice. Thank you!!
     
  2. tpny

    Thread Starter Member

    May 6, 2012
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  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The CRT is normally set up so the three CRT guns emit black and white or Grey Scale, That is not the colour signal, just that the 3 guns on the CRT are working, the colour signal originates from the chroma section.
    Where the problem could be, or between this and the CRT.
    Max.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    I learned a long time ago, dont stick meter probes into the sweep circuits unless your sure what your going to find. there is way more voltage there than normal meters can handle, and its mostly highfrequency ac (rf) that dosnt need a ground to discharge some hurt to you or your meter.,
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You forgot the engineering maxim, "If it ain't broke, don't try to fix it." ;)
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Does the tv have teletext, what is the colour of the white text writing, this will give you a clue as to which gun is down.
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If you colour bars (usually looks green/purple) try resoldering all the legs of that large jungle (chroma) IC. Especially if there looks to be any heat discolouration on the PCB at its legs.

    The fact you already found a heat solder fault on the neck board in your other thread points to the fact the set has run hot for a lot of years and is probably riddled with heat related solder joint faults.
     
  8. tpny

    Thread Starter Member

    May 6, 2012
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    I replaced the raster transistors on the neckboard and still same - two tone pictures. The rgb raster is fine as I see colored text and volumn bars etc.. Just picture comes out in 2 tone. I plan to replace the chroma chip.. Any other possibilities?
     
  9. tom_s

    Member

    Jun 27, 2014
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    been many years since i've worked on a crt, 99 times out of 100 the neck board is riddled with dry/cold joints. i'd go over the neck board again using magnifying glass.

    regards
     
  10. tindel

    Active Member

    Sep 16, 2012
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    Static sounds + Overload on your meter = very high voltage. Probably over 1000V. Sounds to me like you almost had a natural selection event.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    That's about where the OP came in - then started jabbing test prods at the flyback tags on the scan yoke!
     
  12. tom_s

    Member

    Jun 27, 2014
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    the circuit looks close to what your describing. from the schematic, base emitter volts are a little high, collector looks about right.

    just a thought, didn't bump the service switch by mistake?

    edit: as no-one here knows me, it might seem that i'm pointing out obvious, no malice (for lack of a better word) is intended. this may have been the original fault.

    now, i also assume op is using an isolation transformer on the mains?

    and crt nowadays, sometimes better to bite the bullit and pick up a cheap led replacement. picked one up a week ago for $120AUD

    regards
     
    Last edited: Jun 28, 2014
  13. tpny

    Thread Starter Member

    May 6, 2012
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    I replaced the chroma chip and things are well and working again - colors are good. Either the chroma chip faulted or the underside of it on the pcb has bad solder joints. The joints are all brown..
     
  14. tpny

    Thread Starter Member

    May 6, 2012
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    Ya, guess I was asking for trouble.. What is a flyback tag? Thank you!
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    A tag on the scan yoke where the wire with the flyback pulses is soldered on.

    The narrow part of the pulses is a few kV - I think it could be as much as 8kV on the old tube sets.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Flyback transformer or HOT (Horizontal output Transformer) used for a few functions, to provide the 20kv+ to the CRT, drive current to the horizontal scan coils, and often other functions, including crt cathode filament (heater) supply etc.
    Max.
     
  17. tom_s

    Member

    Jun 27, 2014
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    all happy in the end, well done.:)
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Resoldering (see post #7) would most likely have fixed it.

    Only one on ten or so times does the chip need replacing.
     
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